Method of growing shaped crystals



March 13, 1962 Filed April 23, 1958 W. R. RUNYAN METHOD OF GROWINGSHAPED CRYSTALS 2 Sheets-Sheet 1 INVENTOR ATTORNEYS Unite States aterice 3,025,146 METHQD F GRQWLNG SHAPED CRYSTALS Walter R. Runyan, Dallas,Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., acorporation of Delaware Filed Apr. 23, 1958, SenNo. 730,383 8 Claims.(Cl. 23-301) This invention relates to a method of producing shapedarticles of a semiconductor material, and more particularly to a methodof growing shaped germanium or silicon crystals from a melt.

Large crystals of germanium and silicon have been found to be of greatpractical utility as windows and lenses in infrared systems. Thecrystals employed are optically finished to various shapes. This isusually accomplished by growing in conventional fashion a large crystaland grinding away lunwanted regions to produce the desired shape. Inaddition to being excessively time consuming, the technique of grindingresults in the loss of considerable semiconductor material.

It is therefore an object of the present invention to provide a methodof producing shaped crystals of semiconductor material which greatlyreduces the amount of grinding required and thus enables substantialsaving in time and cost of production, particularly as regardssemiconductor material.

The method is particularly adapted to the growing of dome-shapedcrystals of germanium and silicon.

These and other objects and the nature and advantages of the instantinvention will become readily apparent from the following descriptiontaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a view, partly in section, of one form of apparatus that canbe used to carry out the method of the present invention;

FIG. 2 is a view in section showing a plug attached to a seed crystal;

FIG. 3 is a similar view showing the plug about which a shaped crystalhas been grown;

KG. 4 is a similar view showing the shaped crystal for-med in accordancewith the present invention, after the plug has been removed;

FIG. 5 is a view showing an alternative technique for growing a shapedcrystal; and

FIG. 6 is a view similar to FIG. 5 showing the grown crystal.

Referring now to the drawings, a conventional crystal pulling apparatusis shown in FIG. 1 consisting essentially of a graphite crucible 10having a quartz liner 11 enclosed within a housing 12. A REF. coil 13surrounds the crucible 10 and heats it by induction. Inert gas is passedthrough the housing as indicated. A pull rod 15 arranged for combinedrotary and vertically axial movement extends into housing 12 inconventional fashion.

In accordance with the present invention, shaped crystals ofsemiconductor material are grown by attaching a plug 26 of suitableshape and design to a crystal seed 24 mounted in the end of rod 15. Acrystal 25 is grown from a melt in the crucible by operation of theapparatus in accordance with standard practice. The crystal grows on theouter surface of the plug 26 to the thickness and shape desired.

The plug 26 is made of any material substantially inert to thesemiconductor crystal being grown, or at least the surface of plug 26 isprovided with a material substantially inert to the semiconductorcrystal being grown. For example, in the case of germanium, the plug 26can be made of graphite. When growing a crystal of silicon, the plug canbe made of silicon coated with an oxide, silicon nitride or boronnitride. The material selected for the plug must be harmonized with thesemiconductor material especially from the standpoint of thermalcoefiicients of expansion and other related properties.

The plug 26 can be attached to the crystal seed 24 by any suitablemeans. As illustrated in FIG. 2, a hole is drilled in the crystal seed214 into which a rod-like proection 27 on the plug 26 is inserted. Theprojection 27 is shown retained therein by means of a pin 23 passingthrough the crystal seed 24- and the rod-like projection 27. When it isdesired to remove the plug 2 6, the pin 28 is removed, thus permittingthe plug 25' to be removed. Other means can be utilized, such as bycementing the plug 26 to the crystal seed 24 or by any other suitablemeans that will permit subsequent removal of the plug 26.

The plug 26 is provided with an upper surface 29 which corresponds tothe desired shape of crystal to be grown. The drawings illustrate adome-shaped upper surface 29 on the plug 26 that will permit the growthof a dome-shaped crystal 25 as shown in FIG. 4.

An example of the method of growing a crystal in accordance with theteachings of the present invention will now be given. The crucible 10 isfirst provided with a charge of material 16 of the desired composition.A crystal seed 24 is attached to the bottom end of rod 15 byconventional means. A graphite plug 26 having an upper surface 29 of thedesired shape, dome-shaped, for example, is attached to the lower end ofthe crystal seed 24. The crystal seed 24, with the plug 26 attachedthereto is carefully lowered into the molten material 16 until the plug25 is entirely sumberged and the lower end of the crystal seed 24touches the surface of the molten material 16. The rod 15 is thenrotated and moved vertically upwardly slowly. As the rod 15 iswithdrawn, semiconductor material crystallizes onto the crystal seed 24.The temperature is controlled during crystallization in accordance withstandard practice.

After a desired length of crystal has been grown, as for example thelength shown in FIG. 3, the crystal-plug assembly is lifted from themelt. The plug 26 is removed after cooling to yield the dome-shapedcrystal 25 as shown in FIG. 4.

The shape of the plug may be varied as desired to yield crystals of thedesired shape. Beside the configuration shown and previously described,it will be appreciated that it is possible to continue the crystalgrowing process as discussed above, and grow a crystal of semiconductivematerial completely surrounding the plug 26. Under these circumstances,it would be necessary to cut the crystal in half, in order to remove theplug 26. This would have one advantage, namely, if the plug were formedwith a convex lower surface, the lower half of the grown crystal wouldalso have a complementary concave surface.

A further technique for producing shaped crystals is illustrated inFIGS. 5 and 6. This technique for producing crystals of silicon uses asilicon plug, identified by the reference numeral 30. As shown, the plug30 is substantially hemispherical and is provided with a smallprojection 31 substantially centrally of its plane surface. Projection31 is adapted to be connected with the pull rod of the crystal pullingapparatus in a conventional way. The convex surface of the plug 30 iscoated with a layer 32 of material which will inhibit crystal growth onthe convex surface when the plug is inserted into a silicon melt.Suitable materials for this purpose are silicon monoxide and siliconnitride. It will be appreciated, however, that any other materials whichperform the desired function may be used.

The coated plug is introduced into the silicon melt 34 contained in aquartz crucible 33 of a conventional crystal pulling apparatus such asshown in FIG. 1. The melt 34 is characterized by a temperature gradientsuch that the portions of the melt around the periphery of the crucibleand adjacent the surface are the hottest, whereas, the portion of themelt centrally within the crucible and adjacent its bottom is thecoolest. This is due to recognized factors. The silicon material of themelt will commence crystallizing during a normal pulling operation andwill stick to the silicon plug especially about the region of the plugadjacent the surface of the melt. A crystal 35 will be grown onto theplug substantially as shown in FIGURE 6, but will be capable of beingfreed from the silicon plug 30 due to the presence of the layer 32 ofmaterial coating the surface of the plug 30. Removal of the crystal 35can be accomplished in several ways, for example by making a cut bymeans of a diamond wheel as indicated in FIG. 6 by dotted lines 36.

It will be obvious to those schooled in the art that various changes maybe made without departing from the spirit of the invention and thereforethe invention is not limited to what is described in the specificationbut only as indicated in the appended claims.

What is claimed is:

1. The method of growing a shaped single crystal of semiconductormaterial which comprises melting a mass of semiconductor material,introducing into the melted material a single crystal seed portionhaving a shaped plug portion attached thereto, surface of said plugportion being substantially inert to said semiconductor material.withdrawing the single crystal seed portion and attached plug portionfrom the melted material to cause the melted material to crystallizeonto the single crystal seed portion and around the plug portion toproduce a single crystal of said material having a shape complementaryto that of said plug portion, and removing said plug portion from saidgrown single crystal.

2. The method as in claim 1, wherein said single crystal seed portionand melted material are germanium.

3. The method as in claim 2, wherein said plug portion is made ofgraphite.

4. The method as in claim 1, wherein said single crystal seed portionand melted material are silicon.

5. The method as in claim 4, wherein said plug portion is selected fromthe class consisting of silicon, silicon nitride and boron nitride.

6. The method as in claim 1, wherein said plug portion has a dome-shapedupper surface.

7. The method as in claim 3, wherein said plug portion has a dome-shapedupper surface.

8. The method as in claim 5, wherein said plug portion has a dome-shapedupper surface.

References Cited in the tile of this patent UNITED STATES PATENTS463,646 Pease Nov. 24, 1891 1,336,224 Hancock Apr. 6, 1920 2,815,303Smith Dec. 3, 1957 FOREIGN PATENTS 745,037 Great Britain Feb. 15, 19561,025,526 Germany Mar. 6, 1958 OTHER REFERENCES Bell Tel. Lab., et al.Transistor Technology, vol. 1, September 1952, page 75.

1. THE METHOD OF GROWING A SHAPED SINGLE CRYSTAL OF SEMICONDUCTORMATERIAL WHICH COMPRISES MELTING A MASS OF SEMICONDUCTOR MATERIAL,INTRODUCING INTO THE MELTED MATERIAL A SINGLE CRYSTAL SEED PORTIONHAVING A SHAPED PLUG PORTION ATTACHED THERETO, SURFACE OF SAID PLUGPORTION BEING SUBSTANTIALLY INERT TO SAID SEMICONDUCTOR MA TERIAL,WITHDRAWING THE SINGLE CRYSTAL SEED PORTION AND ATTACHED PLUG PORTIONFROM THE MELTED MATERIAL TO CAUSE THE MELTED MATERIAL TO CRYSTALLIZEONTO THE SINGLE CRYSTAL SEED PORTION AND AROUND THE PLUG PORTION TOPRODUCE A SINGLE CRYSTAL OF SAID MATERIAL HAVING A SHAPE COMPLEMENTARYTO THAT OF SAID PLUG PORTIO, AND REMOVING SAID PLUG PORTION FROM SAIDGROWN SINGLE CRYSTAL.